Method of chemically altering a silicon surface and associated electrical devices

ABSTRACT

A method of chemically altering a silicon surface and associated dielectric materials are disclosed.

FIELD

The present disclosure relates generally to a method of chemicallyaltering a silicon surface and electrical devices having the chemicallyaltered silicon surface. The present disclosure particularly relates tothe creation of a dielectric material on a semiconductor deviceutilizing atomic layer deposition.

BACKGROUND

The shrinking of the field-effect device channel length requires anincrease in the capacitance of the gate dielectric in order to achievedesired performance. There are difficulties associated with decreasingthe oxide thickness in a reproducible fashion when that gate dielectricis SiO₂. Leakage currents are unacceptable when the oxide thickness isless than 1.2 nm. An alternative approach has been to deposit high-kdielectrics. However, a difficulty with this approach is that, ingeneral, these materials are mismatched with the underlying siliconlattice. This leads to formation of additional interface states, whichdegrades the device performance. These relatively high dielectricmaterial display temperature sensitivity with respect to micro-crystalformation, migration phenomenon, and relatively low intrinsic dielectricconstants. In addition, these materials are not easy to alter or modify.

SUMMARY

According to one illustrative embodiment, there is provided a method ofchemically altering a silicon surface. The method includes (a) reactinga halide of a first element having only one positive divalent oxidationstate with a hydroxyl group bound to a silicon atom of the siliconsurface so as to chemically couple the first element to the silicon atomof the silicon surface, (b) hydrolyzing a bond between a halogen atomand an atom of the first element so as to generate a hydroxyl groupbound to the atom of the first element, and (c) reacting a halide of asecond element that has a trigonal bipyramidal structure with thehydroxyl group bound to the atom of the first element so as tochemically couple the halide of the second element to the atom of thefirst element.

According to another illustrative embodiment, there is provided anelectronic device. The electronic device includes a silicon substratewith an atom of an element having only one positive divalent oxidationstate chemically coupled to a silicon atom of the silicon substrate. Theelectronic device also includes a trigonal bipyramidal moiety chemicallycoupled to the atom having only one positive divalent oxidation state.

According to yet another illustrative embodiment, there is provided asemiconductor device. The semiconductor device includes a dielectricmaterial which has an atom of an element having only one positivedivalent oxidation state chemically coupled to a silicon atom of asilicon substrate of the dielectric material. The semiconductor devicealso includes a trigonal bipyramidal moiety chemically coupled to theatom of the element having only one positive divalent oxidation state.

DETAILED DESCRIPTION OF THE DISCLOSURE

While the invention is susceptible to various modifications andalternative forms, specific exemplary embodiments thereof have beenshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit the invention to the particular forms disclosed, but on thecontrary, the intention is to cover all modifications, equivalents, andalternatives falling within in the spirit and scope of the invention.

The present disclosure relates to altering a silicon surface so as tocreate a dielectric material on the silicon surface. For example, theaforementioned silicon surface can be defined on an electronic device,such as a semiconductor wafer, by a silicon substrate. Initially thesilicon surface to be altered in accordance with the present disclosureis cleaned with any known appropriate cleaning solution to remove oxidestherefrom. After removing oxides from the silicon surface in theaforementioned manner, the surface is exposed to a hydrolyzingatmosphere such that the silicon atoms on the surface to be altered havea free hydroxyl group covalently bound thereto. For example, contactingthe silicon surface with steam or vaporized hydrogen peroxide in a knownmanner can be utilized to generate free hydroxyl groups covalently boundto the silicon atoms.

After preparing the silicon surface in the above described manner, thehydroxyl groups bound to the silicon atoms are reacted with a halide ofan element that exhibits only one positive divalent oxidation state.What is meant herein by a “halide” is a compound of the type MX_(n),where X is a halogen (e.g. chlorine) and M is another element. Withrespect to elements that exhibit only one positive divalent oxidationstate, examples of such elements are those contained in groups IIA andIIB of the periodic table. For example, calcium of group IIA or cadmiumof group IIB can be utilized as the aforementioned element whichexhibits only one positive divalent oxidation state. To initiate thealteration, a stoichiometric excess of the aforementioned haliderelative to the hydroxyl groups is placed in contact with the siliconsurface under vacuum conditions in a reaction chamber. In particular,the halide is introduced into the reaction chamber in gaseous form sothat the halide contacts the silicon surface. Note that as used herein“vacuum conditions” means that gas is being removed from the reactionchamber so that the pressure in the reaction chamber is less thatatmospheric. The silicon surface (e.g. the silicon surface of anelectrical device such as a semiconductor wafer) is then heated at asufficient temperature (e.g. about 200° C. to about 300° C.) such that(i) the halide reacts with the hydroxyls from the underlying siliconsurface and (ii) the unreacted halide is vaporized and removed from thereaction chamber. An example of this initial alteration of the siliconsurface is schematically illustrated below with the cadmium chloride(CdCl₂):

Note that the unreacted cadmium chloride has a vapor pressure of about 8mTorr at 412° C. and thus can be removed under vacuum conditions withthe hydrogen chloride (HCl) gas product as a by-product of the cadmiumchloride/silicon surface reaction. It should be understood that,preferably, all by-products of the surface reaction, including all ofthe by-products of the surface reaction products described below, shouldbe removed from the reaction chamber as quickly as possible (e.g. via avacuum pump) so as to ensure the surface reaction is driven tocompletion. As illustrated above, the reaction of the halide with thesilicon surface results in the halide being chemically coupled to asilicon atom of the silicon substrate via a covalent bond with theoxygen atom of the hydroxyl group.

After reacting the silicon surface with the halide in the abovedescribed manner, the surface is heated (e.g. to about 200° C.) undervacuum conditions and exposed to vaporized hydrogen peroxide (H₂O₂), ora stream of vaporized water and ammonia (NH₃), or vaporized water (H₂O)so as to hydrolyze bonds between halogen atoms and atoms of the elementthat exhibits only one positive divalent oxidation state and therebygenerate a hydroxyl group bound to atoms of the element. The process ofhydrolyzing bonds between halogen atoms and atoms of the element isschematically illustrated below with hydrogen peroxide:

As illustrated above, hydrolyzing bonds between halogen atoms and atomsof the element results in atoms of the element being covalently bound toa hydroxyl group.

After hydrolyzing bonds between halogen atoms and atoms of the elementas described above, the chemically altered silicon surface is reactedwith a halide of an element that has a trigonal bipyramidal structure.For example, a pentahalide of an element selected from group VB of theperiodic table can be reacted with the silicon surface. Preferably,trigonal bipyramidal halide molecules having a niobium atom or atantalum atom as the central atom of the molecule is reacted with thechemically altered silicon surface. For example, niobium pentachlorideor tantalum pentachloride can be reacted with the silicon surface afterthe above described hydrolysis step. To initiate the reaction, astoichiometric excess of the aforementioned trigonal bipyramidal halidemolecule relative to the hydroxyl groups bound to the element is placedin contact with the silicon surface under vacuum conditions in areaction chamber. In particular, the trigonal bipyramidal halidemolecule is advanced into the reaction chamber so as to contact thesilicon surface. For example, niobium pentachloride heated to 160° C.has a vapor pressure of about 9 mTorr and thus can be advanced into thereaction chamber in a gaseous form and reacted with the silicon surface.As another example, tantalum pentachloride heated to 50° C. has a vaporpressure of about 9 mTorr and thus can be advanced into the reactionchamber in a gaseous form and reacted with the silicon surface. Inparticular, the silicon surface is heated at a sufficient temperature(e.g. about 200° C. to about 300° C.) in the presence of the gaseoustrigonal bipyramidal halide such that (i) the trigonal bipyramidalhalide reacts with the hydroxyls bound to the element and (ii) theunreacted trigonal bipyramidal halide is vaporized and removed from thereaction chamber. It should be understood that the trigonal bipyramidalhalide can be reacted with the hydroxyls bound to the element in thepresence of ammonia at a stoichiometric ratio of about 1:1 with thetrigonal bipyramidal halide if desired. An example of this reaction isschematically illustrated below with the trigonal bipyramidal halideniobium pentachloride:

As illustrated above, the reaction of trigonal bipyramidal halidemolecules with the hydroxyls bound to the element results in a trigonalbipyramidal moiety being chemically coupled to a silicon atom of thesurface via a covalent bond with the oxygen atom bound to the element.

After reacting the chemically altered silicon surface with trigonalbipyramidal halide molecules in the above described manner, bondsbetween halogen atoms and the central atom of the trigonal bipyramidalmoiety are hydrolyzed so as to generate hydroxyl groups bound to thecentral atom. For example, the silicon surface is heated (e.g. fromabout 200° C. to about 300° C.) under vacuum conditions and exposed tovaporized hydrogen peroxide (H₂O₂), or a stream of vaporized water andammonia (NH₃), or vaporized water (H₂O) so as to hydrolyze bonds betweenhalogen atoms and the central atom of the trigonal bipyramidal moiety.An example of this reaction is schematically illustrated below:

As illustrated above, it should be appreciated that heating the siliconsurface in the above described manner also results in the condensationof trigonal bipyramidal moiety hydroxyl groups which are positioned inthe plane that is parallel to the plane defined by the silicon atoms.For example, the hydroxyl of a trigonal bipyramidal moiety which ispositioned in the parallel plane is condensed with a hydroxyl group ofan adjacent trigonal bipyramidal moiety which is also positioned in theparallel plane.

The above described process can then be started over so as to disposeanother layer of dielectric material onto the silicon surface. Forexample, to initiate a second cycle of deposition of a dielectricmaterial onto the silicon surface, as described above, cadmium chloride(CdCl₂) is introduced into the reaction chamber to react with the freehydroxyl groups bound to niobium atoms. For example, cadmium chloride isreacted with the hydroxyl groups extending above the plane which isparallel to the plane defined by the silicon atoms. Thereafter,hydroxyls are introduced onto the cadmium atoms and then the hydroxylsare reacted with trigonal bipyramidal halide molecules with thesubsequent introduction and condensation of hydroxyls on the trigonalbipyramidal moieties.

It should be appreciated that various combinations of dielectric layersare possible. For example, magnesium or calcium can be used in place ofcadmium. In addition, it should be appreciated that, for example, layersof niobate can be alternated with tantalate layers. It should also beunderstood that although the illustration employed a chloride salt, theother halide salts (bromide and iodide) can be used. Thermodynamically,use of the bromide and iodide will not cause any difficulty for thegroup IIA elements but the cadmium (group IIB) iodide might not besufficiently reactive. It should also be understood that the capabilityto vary the layer composition also allows a flexibility in creatingdielectric materials on a silicon surface with a relatively wide rangeof dielectric constants. The dielectric constant can vary between thelow twenties to almost an order of magnitude higher. The dielectricstructure disclosed herein helps to minimize leakage paths since eachniobate or tantalate layer is isolated from another niobate or tantalatelayer by an atomic scale insulator. Accordingly, the methods anddielectric materials described herein help minimize (i) excess interfacestate creation due to structural mismatches and (ii) micro-crystalformation. The methods and dielectric materials described herein also(i) offer ease in modifying the intrinsic construction of a dielectricmaterial and (ii) provide a choice from a range of dielectric constants.

While the disclosure is susceptible to various modifications andalternative forms, specific exemplary embodiments thereof have beenshown by way of example in the drawings and has herein been described indetail. It should be understood, however, that there is no intent tolimit the disclosure to the particular forms disclosed, but on thecontrary, the intention is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the disclosure.

There are a plurality of advantages of the present disclosure arisingfrom the various features of the apparatus and methods described herein.It will be noted that alternative embodiments of the apparatus andmethods of the present disclosure may not include all of the featuresdescribed yet still benefit from at least some of the advantages of suchfeatures. Those of ordinary skill in the art may readily devise theirown implementations of an apparatus and method that incorporate one ormore of the features of the present disclosure and fall within thespirit and scope of the present disclosure.

What is claimed is:
 1. A method of chemically altering a siliconsurface, comprising: (a) reacting a halide of a first element havingonly one positive divalent oxidation state with a hydroxyl group boundto a silicon atom of said silicon surface so as to chemically couplesaid first element to said silicon atom of said silicon surface; (b)hydrolyzing a bond between a halogen atom and an atom of said firstelement so as to generate a hydroxyl group bound to said atom of saidfirst element; and (c) reacting a halide of a second element that has atrigonal bipyramidal structure with said hydroxyl group bound to saidatom of said first element so as to chemically couple said halide ofsaid second element to said atom of said first element.
 2. The method ofclaim 1, further comprising: (d) hydrolyzing a bond between a halogenatom and a central atom of said trigonal bipyramidal halide molecule soas to generate a hydroxyl group bound to said central atom.
 3. Themethod of claim 2, wherein: (d) includes condensing said hydroxyl groupbound to said central atom with a hydroxyl group of an adjacent trigonalbipyramidal molecule.
 4. The method of claim 1, wherein: said element isselected from group IIA of the periodic table.
 5. The method of claim 1,wherein: said element is selected from group IIB of the periodic table.6. The method of claim 1, wherein: said halide includes chlorine.
 7. Themethod of claim 1, wherein: said halide includes cadmium chloride. 8.The method of claim 1, wherein: (a) includes heating said siliconsurface in a reaction chamber to cause unreacted halide of said firstelement to vaporize while maintaining a pressure in said reactionchamber which is sufficiently less than atmospheric pressure to causethe evacuation of vaporized unreacted halide of said first element fromsaid reaction chamber.
 9. The method of claim 1, wherein: (b) includesexposing said silicon surface to a substance selected from the groupconsisting of vaporized hydrogen peroxide, a mixture of vaporized waterand ammonia, or vaporized water.
 10. The method of claim 1, wherein: (c)includes reacting said halide of said second element with said hydroxylgroup bound to said atom of said first element in the presence ofammonia.
 11. The method of claim 1, wherein: said halide of said secondelement has a central atom and said central atom is a niobium atom. 12.The method of claim 11, wherein: said halide of said second element isniobium pentachloride.
 13. The method of claim 1, wherein: said halideof said second element has a central atom and said central atom is atantalum atom.
 14. The method of claim 13, wherein: said halide of saidsecond element is tantalum pentachloride.